I'm guessing you haven't spent much time with SCSI. MS-DOS probably won't benefit much from a U320 controller, and could actually be slower. U320 is meant for PCIX or better, otherwise it'll tie up the whole bus. You won't get more than 133MB/s or 266MB/s (on 64bit or 66mhz PCI) out of it. Best get something like an adaptec 2930. Cache is bit of a joke and as far as I know, only come on raid cards. It slows things down more unless your running a DB on an array. The reason I know all this is I went down that road and got burned. Adaptec 2100S, U320, 32bit PCI, RAID, onboard cache=garbage with really cool knightrider LEDs.
Drivers have nothing to do with boot time. The card needs to have an onboard BIOS ROM to boot.
EDIT: You only need drivers for devices not used at boot time like removable disks, cdroms, scanners, etc. You could get a 29160N to max out your bus if you really wanted to, but if you plan to run Windows 3.1x, you will not be able to run 32-bit disk drivers.
EDIT: You also can't run NT 3.51 on a 29160N. They only supported NT4/95 and up.